1. Field of the Invention
This invention relates to a method for determining in situ the fluid saturation of a fluid-containing subterranean reservoir penetrated by a plurality of wells. More particularly, this invention relates to such a method for determining the fluid saturation in a reservoir which has been subjected to a flooding operation.
2. Description of the Prior Art
In the production of fluids from reservoirs or the injection of fluids into reservoirs, it is often desirable to estimate the relative amounts of the various fluids in place in the reservoir. Such information is helpful in predicting production and/or injection capabilities of the reservoir. This information is especially useful in planning tertiary oil recovery processes which are often expensive and time consuming. In order to design an optimum tertiary recovery process, a knowledge of the amount of water and/or oil present in the reservoir is highly desirable. Various techniques have evolved for determining the amount of a fluid, for example oil, present in a reservoir. The oil content may be expressed conveniently as a percentage of the volume of the pore space existing in the reservoir. The resulting figure is called the "percentage saturation" of oil. The oil content of a reservoir following a secondary recovery operation is called the "residual oil saturation." Residual oil can remain in the reservoir because of a variety of reasons, such as the secondary recovery fluid failing to sweep through certain portions of the reservoir, the particular secondary recovery fluid employed being inefficient, the secondary recovery operation not being carried out for a sufficient length of time, and the like. Much of this residual oil may still be recoverable by further oil recovery techniques. Residual oil saturation is to be distinguished from "irreducible oil saturation" which is the amonnt of oil which cannot be recovered from the reservoir because of capillary forces. The object of a tertiary recovery operation is to recover as much as possible of the residual oil saturation of the reservoir. One technique for determining residual oil saturation involves lowering a logging tool into a well penetrating the reservoir and logging the well. A typical logging tool is capable of examining the reservoir over an area extending from a few inches up to several feet away from the well. In these logging operations, the water saturation can be determined first and the oil saturation then calculated from this value. While reference is made throughout this discussion to "oil saturation," it is to be understood that these same procedures can also be used to determine "water saturation," "hydrocarbon saturation," and "gas saturation."
Various techniques for determining residual oil saturation using different logging tools are well known in the art. Broadly, any reservoir-property-determining, log-measurement means can be employed. The usual logs are the electrical resistivity pulsed neutron and electromagnetic propagation logs. The various techniques employed generally include logging the reservoir a first time, injecting into the reservoir a fluid or fluids which alter the water in the reservoir, the oil in the reservoir, or both the water and oil, and logging the reservoir a second time. Additional sequences of fluid injection followed by logging can be employed.
U.S. Pat. No. 3,562,523 to Richardson et al, describes a log-inject-log method for determining the residual oil saturation in a reservoir which has undergone a water drive or water-flooding wherein first a nuclear logging tool is positioned opposite the reservoir and the thermal neutron decay is measured. The rate of neutron decay is dependent upon the nuclear capture cross section of the formation rock, the capture cross section of the fluids contained within the reservoir rock, and the volumetric fractions of the rock and the fluids. Next, the aqueous liquid present in the reservoir within the radius of investigation of the logging tool is displaced by a second aqueous liquid having a materially different capture cross section than the aqueous liquid originally in place without disturbing the oil phase present in the reservoir. The thermal neutron decay is measured again. The water saturation is determined from the difference between the two thermal neutron decay measurements. The residual oil saturation is determined by difference. Previous procedures using a resistivity logging tool are discussed.
U.S. Pat. No. 3,631,245 to Jorden, Jr. describes a process similar to the above except that rather than displacing the aqueous phase originally in place, in indigenous oil phase is removed and replaced with an aqueous phase substantially equivalent in composition to the aqueous phase originally in place.
During a conventional secondary recovery operation, large amounts of a secondary recovery fluid are injected into the reservoir via one or more injection wells and fluids are withdrawn from the reservoir via one or more production wells. A study of the flow pattern of secondary recovery fluids passing through a reservoir shows that a relatively large amount of fluid passes through the portion of the reservoir immediately surrounding the spaced injection and production wells as compared with the amount passing through the bulk of reservoir between the wells. Thus, the fluid content of the reservoir in the vicinity of a well previously used in a secondary recovery process often will not be representative of the fluid content throughout the bulk of the reservoir. This means that logging such a well will provide a result which is not representative of the saturation throughout the bulk of the reservoir.
U.S. Pat. No. 3,757,575 to Murphy et al. describes a method for determining the residual oil saturation of a reservoir by a log-inject-log technique using either an electrical resistivity log or a Thermal Decay Time Log, a trademark of Schlumberger Well Services. A well penetrating the reservoir is first prepared in a manner such that the gas, oil and water saturation conditions adjacent the wellbore are representative of those out in the reservoir remote from the well. This is accomplished by controlled production of the well, i.e., producing the well at a very slow rate so as to have a small pressure drop between the well and the reservoir remote from the well. A first log is run. A first drive fluid is injected to reduce the oil saturation to zero in the volume of reservoir investigated by the log. A second drive fluid having similar characteristics to the formation water is injected to displace the first drive fluid. A second log is run.
Thus, Murphy et al. recognize that in logging wells to determine residual oil saturation, the reservoir adjacent the wellbore must be prepared so that the fluid saturation conditions there are representative of those on out in the bulk of the reservoir. However, they propose a solution to this problem in which the problem must be recognized well in advance of the time it is desired to determine the residual oil saturation. The method can be quite time consuming. During the time the well must be produced at a very slow rate, a large quantity of potential oil production will be lost. In contrast, the method of the instant invention can be carried out in a relatively short period of time regardless of a well's prior production and/or injection history.
Accordingly, a principal object of this invention is to provide an improved process for determining the fluid saturation, e.g., the oil saturation, in a subterranean oil-bearing reservoir.
Another object of the invention is to provide such a process which is operable in a reservoir which has been flooded.
Still another object of the invention is to provide such a process which is operable in a well penetrating such a reservoir wherein the fluid saturation in the reservoir immediately adjacent the well is different from that in the bulk of the formation.
A further object of the invention is to provide a process for determining the residual oil saturation of a reservoir which has been waterflooded, which saturation is representative of that in the main body of the reservoir, in equivalent rock quality, substantially removed from a well penetrating the reservoir.
Other objects, advantages and features of this invention will be apparent from the following detailed description.